1. Technical Field
The present invention relates to tunable filters, and finds particular application in the use of gratings in tuning optical sources.
2. Description of Related Art
Semiconductor laser diodes are known as optical sources. Various techniques are used to obtain single mode, narrow linewidth operation which is desirable in applications such as communications. For instance, unmodified edge emitting laser diodes typically operate with several longitudinal modes lasing simultaneously, leading to low coherence and large linewidths. A technique known for use with edge emitting laser diodes is to use a grating to act as a wavelength filter in providing feedback to the lasing cavity to obtain single mode operation. Examples are the distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers in which a grating extends at least partially along the length of the lasing cavity, or a neighbouring cavity, being formed for instance in the surface of a substrate supporting the lasing cavity and any neighbouring cavity.
An external cavity laser (ECL) is another known arrangement which uses wavelength selective feedback to obtain single mode operation. In this case, the lasing cavity is coupled via an end facet to a further cavity, usually in free space, that contains a wavelength-selective feedback mechanism to provide feedback to the lasing cavity. The wavelength-selective feedback mechanism typically comprises individually mounted wavelength-selective optics in the free space. In the ECL, the “lasing” cavity is sometimes described as a “gain” cavity rather than a lasing cavity as the end facet is anti-reflection coated, giving the laser diode the construction of a gain element rather than a laser. DFB/DBR and ECL lasers differ in more than one way. Firstly, DFB/DBR lasers are generally monolithic, their lasing and feedback sections being fabricated on the same substrate. This is in contrast to an ECL in which the wavelength-selective optics are usually fabricated separately and mounted in relation to the lasing cavity in a separate mounting operation. Secondly, wavelength selection in an ECL is generally provided by some sort of filtering device having filtering elements arranged to provide a plane or surface which is transverse to the optical path of radiation from the lasing cavity and plays a part in defining the length of the external cavity, often in free space. This is in contrast to the distributed feedback arrangement in which the wavelength selection mechanism is distributed longitudinally along the optical path of the radiation in a waveguide. In an ECL, wavelength selection can be provided for instance by a diffraction grating operating in reflection mode and providing a facet in or of the external cavity. The facet may uniquely, or in combination with one or more further feedback elements such as a mirror, define the physical length of the external cavity by providing a discrete change in direction in the optical path. In a DFB/DBR arrangement, wavelength selection is provided at least predominantly by a longitudinally extending, distributed grating such as a distributed Bragg reflector which does not play a part in defining the length of a cavity but instead is part of a waveguiding structure.
In the above, reference is made to a diffraction grating operating “in reflection mode”. This can generally be taken to mean that the radiation incident upon the grating is diffracted by it to the same side of the grating as it was incident upon it. Thus a grating device operating in reflection mode does not have to be transparent to the optical radiation throughout its thickness. In contrast, a diffraction grating device operating “in transmission mode” is generally transparent to the optical radiation to at least some degree throughout its thickness. Thus it can be used to diffract the incident radiation to either side of the grating. In this case, a portion of the incident radiation can be collected on a different side of the grating from the side on which it was incident.
Lasers are also known which are tunable. Tunable optical sources are required in optical communications systems, cable television systems, local area networks and measurement equipment. For instance in wavelength division multiplexing as used in communications it is necessary to provide optical sources which can operate at distinguishable wavelengths. Although an array of separate devices can be used, each tuned to one of the wavelengths, it becomes expensive to maintain a supply of backup lasers since there has to be a backup laser for every device in the array. In this scenario, it has been recognised that it is preferable to have a tunable laser as backup which can be substituted for some, or indeed any, of the devices in the array. Tunable lasers can also provide significant improvement in local wavelength usage so that optical fibre carriers can be used more flexibly in a communications network with less dependence on centralised intelligence.
In communications, it would be desirable to have a source tunable over the low loss or low dispersion bandwidth windows of an optical fibre for communication. Long distance communication is generally centred on 1310 nm and 1550 nm. In short distance communications such as Local Area Networks (LANs), the equivalent bandwidth window might be centred on 650 nm or 850 nm.
Single mode semi-conductor lasers have been used as tunable sources. For instance, distributed feedback (DFB) lasers have been used but have had a limited tuning range, of the order of 15 nm. This reduces their usefulness in communications. For instance, the International Telecommunications Union (ITU) band of optical channels centred nominally on 1550 nm covers a tuning range of the order of 30 nm.
ECL lasers have also been used as tunable sources. One known configuration uses a diffraction grating in the external cavity to filter a selected wavelength for feedback to the lasing cavity. This is the Littrow configuration. The selected wavelength is diffracted back along the same path as it is incident on the grating so that the grating can provide an end facet of the external cavity in the manner of a retro-reflecting mirror. In the simplest case, wavelength tuning is achieved by controlling the angle of the grating to the incident beam axis. This determines the wavelength diffracted back to the lasing cavity and thus the lasing wavelength.
In tunable sources of this general type, in which feedback is provided by reflection or by a diffraction grating operating in reflection mode, “mode hopping” can arise. This is due to the fact that there will be more than one resonant longitudinal mode for the electromagnetic radiation along the optical path in which oscillation is occurring. To prevent mode-hopping, tuning without interruption of the phase of oscillation, or so-called phase-continuous tuning, should be achieved. A way to do that is to keep the number of half-wavelengths in the optical path in the external cavity constant as the wavelength is tuned. There are known techniques to prevent mode hopping.
In the Littrow arrangement, as mentioned above, tuning is achieved by adjusting the position of the grating but this requires great accuracy. Additionally, if there is a risk of mode hopping for instance because the desired tunable range is large enough, this has to be counteracted. The way this has been done is to move the grating to adjust the optical path length at the same time as carrying out tuning. The various moving parts involved in the optical path are difficult to align in manufacture and to maintain through the working life of the laser and the size of the overall configuration can be simply too large for many applications, being in some cases of the order of tens of centimeters.
There are various aspects of existing tunable sources which could be improved. There is a trade-off between tunable range and power. Some lasers can be configured to replace any of the lasers in today's 40-channel wavelength division multiplexed communication systems but they won't sustain very long transmission distances. Other lasers deliver the power but are not tunable over a wide enough range. Manufacturing costs can be high as some tunable sources are at the edge of what can be done in semiconductor technology and reliability and control are often a problem as the characteristics of individual lasers vary and every one then has to be characterised for use.